Apparatus for decomposing ammonia



June 20, 1933.

cs. w. BURKE 1,915,120

APPARATUS FOR DECOMPOSING AMMONIA Filed Aug. 1, 1930 GAS l/VLET GEORGE W BURKE INVENTOR TTORNEY Patented June 20, 1933 UNITED STATES PATENT OFFICE GEORGE W. BURKE, 0F WILMINGTON, DELAWARE, ASSI GNOR, BY MESNE ASSIGNMENTS,

TO E. I. DU PONT DE NEMOURS & COMPANY, CF VTILMINGTON, DELAWARE, A GOR- PORATION OF DELAWARE APPARATUS FOR DECOMPOSING AMMONIA Application filed August 1, 1930. Serial No. 472,328,

This invention relates to the manufacture of gaseous mixtures of hydrogen and nitrogen by the thermal decomposition or cracking of ammonia.

It has only recently been appreciated that the wide application of the direct synthesis to the manufacture of ammonia has rendered that material so generally available at moderate prices as to make it, for many purposesa very convenient and economical source of hydrogen, nitrogen, or mixtures of these gases. Ammonia can readily be decomposed into hydrogen and nitrogen by passage in the gaseous state and at an elevated temperature over a suitable catalyst. :Besearch directly to the commercial development of this method of gas generation has shown important difficulties rising primarily from the need for a thermally more eiiicient method and an inexpensive, compact, and portable equipment which can be employed economically for cracking ammonia. 7

It is accordingly an object of the invention to provide an improved process for the thermal decomposition or cracking of ammonia characterized by an increased thermal efficiency and smoothness of operation.

It is a further object of the invention to provide an improved method for volatilizing the ammonia which is to be cracked. The improved process which is considered hereinafter is described and claimed in my copending application Ser. No. 622,613.

An additional object of the invention is to provide animproved form of cracking apparatus.

These and other objects of the invention will be more fully understood by reference to the following specification, in which the preferred embodiments are described, and to the accompanying drawing, in which Figure 1 is a sectional view of an apparatus for producing ammonia gas from liquid ammonia, the apparatus comprising a casing 21,

1 provided with cover plates 22 and 23, liquid ammonia inlet land gaseous ammoniaoutlet 2, and enclosing'a heating coil 4, by means of which liquid ammonia is volatilized and preheated at the expense of the heat in the gaseous, products of the dGCOIl'lPOSltlOIl; and

Figure 2 is a view, also in section, of the cracker, consisting of a casing 7, with cover plates 12 and 13 and enclosing tubular members 8, 9, and 14, so disposed as to form annular catalyst receptacles 15 and 16 and a heating chamber 17.

Referring to Figure 1, in operating the apparatus, liquid ammonia, conveniently derived directly from an ordinary cylinder of commercial ammonia, is delivered under its own pressure to the inlet 1. The hot products of the decomposition, efiected in the cracker of Figure 2, in traversing thev coil 4 volatilize the liquid ammonia and preheat the resultant gas, which leaves via the outlet 2 and enters the cracker by the inlet 6. Entering the annular space 15 the ammonia,

contacts with catalyst therein, and the gas being further heated by thermal exchange with previous portions of gas in the passage 16, cracking is initiated almost at once and is continued and completed in the second catalyst space 16. Here the gases flow in the opposite direction and in heat exchange with the gases in' the passage 17. Leaving 16, via theorifices in the ring 24, which rests on rods 25, they gases again reverse the direction of flow to pass through the passage 17 where they contact with the electrical heatingcoil- 10, wound upon the alundum tube 18 which is supported by the tubular member 14. The hot decomposition prodnote leave the cracker by the outlet 19 whence they are delivered to the inlet 3 of the coil 4 of the vaporizer, Figure 1, from which they are withdrawn through outlet 5.

The catalyst employed in the apparatus may be any catalyst active for the decomposition of ammonia. This may consist .'of granular iron, obtained, for example, by the reduction of fused iron oxide, to which promoters, such as aluminum oxide, magnesium oxide, zirconium oxide, etc., may advantageouslybe added. A particularly suitable catalyst may be made by mixing 3% by weight of magnesium oxide and 3% by weight of aluminum oxide to pure ferroso ferrous oxide. Melt the mixture, cool, crush to 814 mesh size,'and reduce with hydrogen at a temperature within the range of 300- .source of heat would berequired. By the 350 C. In selecting the raw materials and in preparing the catalyst care should be taken to avoid contamination by sulphur compounds as well as other catalyst poisons.

While the invention is not limited to the use of any specific temperature and the temperature best for the purpose will vary with the particular catalyst employed, it may be stated that with the catalyst above described very satisfactory results are obtained when the latter is maintained at a temperature of about 550650 G. More specifically, it is preferable that the temperature of the hottest part of the catalyst, which will be from one-half to two-thirds the way up the passage 16, should be about 600. With anapparatus of the character described and shown in thedrawing, the casing 7 being a standard six inch wrought iron pipe, 14%

inches long, and the members 8, 9, and '14,

being l, 2%, and 1 inchwrought iron'pipe respectively, and with a 2100-2200 watt nichrome heating element, 150 cu. ft. of hydrogen-nitrogen mixture were obtained per hour when the ammonia'inlet flow was adjusted so that the gases leaving 19 were substantially free from ammonia.

Employing the procedure and apparatus described the heat exchange between the gases in the cracker produces a relatively uniform temperature and hence a more efficient operation. Thegases admitted to the space 15 are rapidly brought to reaction temperature by thermal exchange with the gases in'space 16. As a consequence in an apparatus of this kind I have succeeded inobtaining substantially 90% decomposition of the ammonia in the space 15, the remaining 10% being then readily efii'ected in space 16. The centrally disposed heating coil 10, being subjected to contact only with ammoniafree gases is not corroded as would otherwise be the case and being insulated by the gases in the surrounding annular catalyst chambers, operates efliciently and economically. 7

It will also be observed that by utilizing the heat in the gaseous products to volatilize the ammonia that is to be decomposed the over-allthermal efiiciency of the process is greatly improved, and the apparatus considerably simplified. Otherwise a separate present arrangement the sensible and otherwise waste heat of the products is used to volatilize the liquid ammonia and preheat the resultant gas, the introduction of this preliminarily warmed gas to the catalyst making possible a more efiicient, utilization of the latter). The arrangement also contributes tosmoothness and uniformity of operation by correcting for slight variations in temperature due to I extraneous causes. Thus, in the event of a temporary rise in temperature of the decomposition chamber the rate of volatilization of ammonia will automatically increase and the larger quantity of ammonia delivered through inlet 6 will tend to retard the endothermic decomposition reaction. Similarly a momentary drop in temperature will automatically be corrected by a decrease in the rate of delivery of ammoniato the cracker.

In practice it is desirable to provide insulation, not shown, upon the casings 21 and 7 as well as the connecting pipes carrying hot gases. Valves, also not shown, are employed for regulating the delivery of the fluids to and from the apparatus.

It will be apparent that the method and apparatus hereinbefore described is well adapted for the eflicient cracking of ammonia on a large'or small scale and the apparatus, being readily constructed in a compact and portable form, is especially suited for industrial uses, such-as welding operations, for example, where hydrogen-containing gases are required more or less intermittently and in fluctuating quantities at different locations.

Various changes may be made in the method and apparatus described without departing from the invention or sacrificing any of the advantages thereof.

Iclaim:

1. In an apparatus for producing a nitrogen-hydrogen mixture by cracking ammonia the combination of a tubular casing and a plurality of concentric tubes fixed thereto to form annular chambers containing ammoniacracking catalyst'and surrounding an inner heating chamber; inlet for the ammonia gas communicating directly with the outer concentric tube containing the catalyst; a removable closure at each end of said casing, one closure carrying an electric heater disposed in said heating chamber and the other carrying a gas outlet communicating directly with said heating chamber.

2. In an apparatus for producing a nitrogen-hydrogen mixture by cracking ammonia the combination of a cracking chamber containing ammonia-cracking catalyst and means for supplying heat thereto, with gas conduits leading respectively to and from a separate ammonia-vaporizing chamber, the latter including heat exchange means adapted to convey the hot gases from the decomposition chamber in heat exchange relation with li uid ammonia in said vaporizing chamber an means for introducing vapor ized ammonia directly from the ammonia vaporizing chamber to the cracking chamber.

3. In an apparatus'for producing a nitrogen hydrogen mixture by cracking ammonia the combination of a cracking chamber containing ammonia-cracking catalyst and means for supplying heat thereto, with gas conduits leading, respectively, to and from a separate ammonia vaporizing chamber,

the latter including a casing provided with an inlet for liquid ammonia and an outlet for gaseous ammonia and enclosing a helical tube connected at the lower end to the gas outlet of the cracking chamber and at the upper end to the gas inlet of the cracking chamber, and means for introducing vaporized ammonia directly from the ammonia vaporizing chamber to the cracking chamber.

4-. In an apparatus for thermal decomposition of ammonia the combination of a plurality of concentric annular catalyst chambers, containing ammonia-decomposing catalyst, an inlet for ammonia gas communicating directly with the outermost catalyst chamber, the innermost catalyst chamber enclosing and communicating with a heating chamber provided with a heater, the latter chamber communicating directly with the outlet for the cracked ammonia gases.

5. In an apparatus for the thermal decompositon of ammonia the combination of an annular catalyst chamber, containing ammonia-decomposing catalyst, an inlet for ammonia gas provided at one end thereof, the catalyst chamber communicating at the other end with and enclosing a second annular catalyst chamber containing ammonia-decomposing catalyst, both catalyst chambers being in heat exchange relation, the second catalyst chamber also communicating with, and enclosing a heating chamber containing a heater, the heating chamber communicating directly with a gas outlet.

In testimony whereof, I afiix my signature.

GEORGE W. BURKE. 

